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Breeding Season for Beginners: What to Expect Mares to Do

Dr. David W. Freeman, OSU Extension Equine Specialist

Those of us that have been around ‘for a few years’ sometimes forget that many horse owners are new to the horse breeding business. Many owners become involved with breeding horses as an after thought. It just seems to follow that once you own horses for a while, you will want to ‘raise a baby’. Welcome to a whole new world of decisions and important things to know. A few of the more frequently asked questions are provided below.

When can I start breeding my mare?

Most mares are not bred intentionally until their three-year-old year, although a small number of people will breed two-year-olds. Mares will reach puberty much earlier than that age. They usually start cycling soon after their yearling birth date. As such, it is important to keep stallions away from young mares until the intended age of breeding. Mares will begin to cycle once they reach puberty. Unless bred, mares will cycle through this estrous season on the average of every 21 days. The estrus season is the time of year that mares are reproductively active.

Mares are long day breeders. Under natural conditions, they are not reproductively active during the season of the year with short daylight hours, i.e. late fall through early spring. Once the daylight portion of the days lengthen in late spring and summer, mares start the estrous season. They will remain reproductively active until bred or when they move into the anestrous season, or the time of year they naturally do not cycle.

I want to breed her so I can have an ‘early foal’. What can I do to ‘make her come in early’?

Some people confuse the veterinary therapies used to alter the mare’s individual estrus cycle once in season with what can, or can’t be done, to alter the time a mare moves into the estrous season. Drug therapies to hasten the onset of the estrous season are limited at best.

There is, however, an easily conducted management tool. It is extended daylight. Mares are long day breeders. Given enough time and regularity of schedule, artificially extending day light with a lighting source will move mares into the estrous season. In order to work, lighting programs must be administered for 60 to 90 days before the time you intend to breed her. Lighting programs must be administered so the mare is ‘under lights’ every day. Lighting programs should allow for 16 hours of total light perception per day, so if the sun comes up at 7 a.m., artificial lighting should come on one half hour before sundown, and continue until 11 p.m. Lighting must be regular and controlled; missing several days in a role may negate the effect. Leaving lights on 24 hours a day will negate the effect. Most people with a small number of mares will simply put a timer on stall lights to manage extended lighting programs.

So I think she is ‘in season’, when do I breed her?

The estrous cycles that a mare goes through will consist of days that she shows estrus, or heat, and days when she is in diestrus, or in between heat days. Once she is in the estrous season, she cycles, which means she is at some stage of growing, ovulating or repairing follicles. A follicle is the part of an ovary that houses the mare’s egg. Even though there is a lot of variation on how long each cycle lasts, a good average is 21 days from ovulation to ovulation. In between the times of ovulation, a mare’s ovaries are developing new follicles. It takes these follicles several days to mature, and during most of this time, the mare is not receptive to breeding or ‘showing signs of heat’. This period is diestrus, which lasts about 16 days on the average.

Once enough follicular growth happens, a mare will start to show signs of heat: she shows interest in stallions, frequently urinates in the presence of teasing stallions, etc. This time period, what we commonly call ‘in-heat’, will last about 4 or 5 days. It is toward the last day of this period that a mare actually ovulates. For best chances of breeding, the mare must be bred sometime between a day and half to up to the time she ovulates. This usually results in a stallion manager timing breedings so mares are bred when she shows signs of heat, every other day until she ‘goes out of heat’. The better the diagnostics, i.e. palpation of the ovaries or visual imaging by ultrasound, the better knowledge of the time of ovulation.

I think she’s bred, how do I know?

With visual imaging, veterinarians might be able to detect pregnancy as early as 10 to 14 days with a fair amount of accuracy. Palpation usually requires the embryo to be a little older, hence, larger. If these diagnostics are not done, the mare can be checked for her receptivity to stallions after a couple of weeks to see if she is still cycling. Once pregnant, she will not build follicles to ovulate or show the typical signs of heat. Of course, it is advisable to recheck her pregnancy a couple of times later to ensure she maintained it.

She didn’t breed, what can I do?

It is normal for mares to be bred through a couple of breeding cycles before they become pregnant, but if this continues, it could be a sign of problems. The more problems, the more diagnostics are needed. The best check is to determine whether or not she actually ovulated or not. It is common for a mare to show signs of heat but not build ovulatory follicles early in the estrous season. Older mares may do the same throughout the season. Any mare might have these troubles, so you may need to increase your diagnostic abilities to insure she ovulated. There are a veterinary therapies that may help mares with these problems, as there are for mares that may have other problems such as uterine infections.

All of these questions and answers are well and good to discuss, however, the biggest expectation is to expect your mare to respond differently to any ‘average’ or general recommendation that we can give. Mares are very individualist in their breeding behavior and physiology. I’ve often said that we read the books on what is suppose to happen, the mare never does. Point is that the more past experience you have with your mare, the more success you will have in knowing what to expect from her in the future.